avformat/id3v2: support buggy id3v2.3 tag length in id3v2.4

Some encoders do not use syncsafe sizes in v2.4 id3 tags. Check the next
tag to try to choose between the two.

Fixes ticket #4003
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/id3v2.c

@@ -170,6 +170,48 @@ static unsigned int get_size(AVIOContext *s, int len)
     return v;
 }
 
+static unsigned int size_to_syncsafe(unsigned int size)
+{
+    return (((size) & (0x7f <<  0)) >> 0) +
+           (((size) & (0x7f <<  8)) >> 1) +
+           (((size) & (0x7f << 16)) >> 2) +
+           (((size) & (0x7f << 24)) >> 3);
+}
+
+/* No real verification, only check that the tag consists of
+ * a combination of capital alpha-numerical characters */
+static int is_tag(const char *buf, unsigned int len)
+{
+    if (!len)
+        return 0;
+
+    while (len--)
+        if ((buf[len] < 'A' ||
+             buf[len] > 'Z') &&
+            (buf[len] < '0' ||
+             buf[len] > '9'))
+            return 0;
+
+    return 1;
+}
+
+/**
+ * Return 1 if the tag of length len at the given offset is valid, 0 if not, -1 on error
+ */
+static int check_tag(AVIOContext *s, int offset, unsigned int len)
+{
+    char tag[4];
+
+    if (len > 4 ||
+        avio_seek(s, offset, SEEK_SET) < 0 ||
+        avio_read(s, tag, len) < len)
+        return -1;
+    else if (!AV_RB32(tag) || is_tag(tag, len))
+        return 1;
+
+    return 0;
+}
+
 /**
  * Free GEOB type extra metadata.
  */
@@ -734,8 +776,26 @@ static void id3v2_parse(AVIOContext *pb, AVDictionary **metadata,
             tag[4] = 0;
             if (version == 3) {
                 tlen = avio_rb32(pb);
-            } else
-                tlen = get_size(pb, 4);
+            } else {
+                /* some encoders incorrectly uses v3 sizes instead of syncsafe ones
+                 * so check the next tag to see which one to use */
+                tlen = avio_rb32(pb);
+                if (tlen > 0x7f) {
+                    if (tlen < len) {
+                        int64_t cur = avio_tell(pb);
+
+                        if (ffio_ensure_seekback(pb, 2 /* tflags */ + tlen + 4 /* next tag */))
+                            break;
+
+                        if (check_tag(pb, cur + 2 + size_to_syncsafe(tlen), 4) == 1)
+                            tlen = size_to_syncsafe(tlen);
+                        else if (check_tag(pb, cur + 2 + tlen, 4) != 1)
+                            break;
+                        avio_seek(pb, cur, SEEK_SET);
+                    } else
+                        tlen = size_to_syncsafe(tlen);
+                }
+            }
             tflags  = avio_rb16(pb);
             tunsync = tflags & ID3v2_FLAG_UNSYNCH;
         } else {